Acid gas removal from various gas streams, and especially removal of carbon dioxide, sulfur dioxide, and hydrogen sulfide from natural gas streams has become increasingly important as regulations for emission of acid gases have become more and more stringent. There are numerous processes for acid gas removal known in the art, and physical solvents are often preferred where feed gas pressures are relatively high (i.e., above 200 psig). The physical absorption of a particular acid gas predominantly depends upon use of solvents having selective solubility for the acid gas (e.g., CO2, or H2S), and is typically further dependent upon pressure and temperature of the solvent and raw gas.
For example, methanol may be employed as a low-boiling organic physical solvent, as exemplified in U.S. Pat. No. 2,863,527. However, the energy input requirements for cooling are relatively high, and the process generally exhibits greater than desired methane and methane absorption, thereby necessitating large energy inputs for recompression and recovery. Alternatively, physical solvents may be operated at ambient or slightly below ambient temperatures, including propylene carbonates as described in U.S. Pat. No. 2,926,751 and those using N-methylpyrrolidone or glycol ethers as described in U.S. Pat. No. 3,505,784. While such solvents may advantageously reduce cooling requirements, at least some of the propylene carbonate-based absorption processes are limited to absorption pressures of less than 1,000 psi (i.e., to sub-critical pressure). In further known methods, physical solvents may also include ethers of polyethylene glycol dimethylethers, and specifically dimethoxytetraethylene glycol as shown in U.S. Pat. No. 2,649,166, or N-substituted morpholine as described in U.S. Pat. No. 3,773,896.
While use of physical solvents avoids at least some of the problems associated with alternative acid gas removal processes (e.g., chemical solvents and/or membranes), various difficulties generally persist. Among other things, as the water content in the solvent increases, freezing may occur in the solvent circuit, thus necessitating a relatively high operating temperature and thereby reducing the efficiency of the absorption process. Furthermore, regeneration of physical solvents requires in many instances steam or external heat to produce a lean solvent suitable for removal of acid gas to the ppm level. Such solvent regeneration is conceptually relatively simple. Typically, rich solvent is successively flashed to lower pressures, and in many instances, further processed in a regenerator that heats the flashed solvent using a steam or fuel fired heater. The so generated heated lean solvent is then cooled (e.g., using external refrigeration) and pumped to the absorber.
In such processes, as carbon dioxide is absorbed by the solvent, the heat of solution of carbon dioxide increases the solvent temperature resulting in a top-to-bottom increasing temperature profile across the absorber. Consequently, one limitation of physical absorption lies in the relatively high absorber bottom temperature, which limits carbon dioxide absorption capacity of the solvent. To overcome the problems associated with limited absorption capacity, the solvent circulation rate may be increased. However, increase in solvent circulation significantly increases refrigeration costs and energy consumption for pumping the solvent. Worse yet, high solvent circulation of known solvent processes will lead to increased loss of methane and hydrocarbons (due to co-absorption).
Alternatively, a stripping column (optionally equipped with a vacuum pump) may be employed as a regenerator for the solvent, as exemplified in an acid gas removal plant shown in U.S. Pat. No. 3,252,269 to Woertz. While such systems often reduce the energy demands for heating, operation of the stripper nevertheless requires some energy (e.g., vacuum pump for vacuum stripper, or heater for atmospheric stripper).
Thus, although there are numerous processes for acid gas removal with physical solvents known in the art, all or almost all of them suffer one or more disadvantages. Most significantly, in numerous known systems, the solvent circulation rate is relatively high, among other factors, due to sub-optimal solvent loading and sub-optimal stripping. Therefore, there is still a need for improved configurations and methods for acid gas removal from a feed gas using a physical solvent.